B sabouni



Jan. 31, 1956 B. B. SABOUNI ELECTRICAL HEATING UNIT AND METHOD OF MAKINGSAME Filed March 16, 1953 2 m2 M wflwR m r// m m e 2/ 9 3 4 g w an f it:L0 r m Vl BAH/OJ 13. 52:50AM

ATTORNE Y5 nited States Patent ELECTRICAL HEATING UNIT AND METHOD OFMAKING SAME Bahidj B. Sabouni, Oakland, Calif. Application March 16,1953, Serial No. 342,541 8 Claims. (Cl. 201-64) This invention relatesto a new and improved electrical heating unit and a method of makingsame.

The embodiment of the heating unit illustrated in the drawings, and tobe described more fully hereinafter, is shown as comprising a solidcasting of heat conductive material such as aluminum alloy, a Wireframe, preferably made of the same material as the casting, and aninsulated resistance wire coiled around the frame in zig-zag pattern,and embedded within the casting. The insulation around the resistancewire comprises a flexible braid of glass strands or yarn.

Although there has heretofore been developed and used various types ofheating units incorporating insulated resistance Wires, one such type inextensive use today comprises an armour sheathed wire generally knownand referred to in the trade as Calrods. This type of unit will usuallycomprise a steel jacket or tubing encasing a resistance Wire which iselectrically insulated from the walls of the metal tubing by compressedmagnesium oxide or like material. Although these units give reasonablysatisfactory and eflicient service in many applications in the electricheating field, they are, generally speaking, relatively expensive tomanufacture. In this connection, the manufacture of Calrods requiresrelatively large and expensive manufacturing facilities combined with ahigh degree of production skill. Furthermore, although such units arecommercially available in a plurality of more or less standard sizes andshapes, manufacturers and fabricators in the heating field generallyfind it to be a time consuming and expensive process to obtain suchunits in non-standard designs for special heating applications.Moreover, because such units by their nature consist of round orpartially flattened tubes or jackets their use in applications whereheat is to be transferred principally by conduction from the units tosolid articles placed in direct contact therewith may involve verysubstantial and undue heat losses. Thus, for example, a conventionalelectric cooking element consisting of a spirally coiled Calrod willinherently suffer substantial heat loss by virtue of air circulationaround and through the coils, and because only a minor fraction of thetotal exterior areas of the coiled rods are in direct contact with thebottom of the cooking vessel placed there upon.

It is a principal object of the present invention to provide an electricheating unit which minimizes to a great extent all of the abovementioned disadvantages and difliculties attendant in the manufactureand use of Calrods or like conventional units. More specifically, it hasbeen determined that a heating unit constructed in accordance with thepresent invention can be manufacured at considerably less cost than amore conventional armour sheathed unit of substantially the same heatout-put. In addition, heating units of the present type can beconstructed out of commercially available materials and according torelatively simple and well-known manufacturing techniques in an almostlimitless number of different designs, shapes and sizes, and withpractically any desired heat out-put. In this connection, and as willmore fully hereinafter appear, the present heating unit is particularlysusceptible to variation and flexibility of designfar more so than aremore conventional Calrods for instance.

It is also true that a heating unit embodying the present invention canbe constructed so as to reduce to a minimum heat loss caused by airconvection. In this connection the present heating unit may be made as asolid, relatively flat casting of practically any desired shape or size.Thus, if such a unit be employed as a cooking element on a stove, it isreadily seen that the flat surface of the unit would insure maximumdirect heat contact transfer between the practically operative orcommercially successful. More specifically, it has been variouslyproposed to insulate a resistant wire with fused glass, porcelainenamel, fire clay and/ or mica, as examples, and then to pour moltenheat-conductive metal around the resistant Wire thus insulated so as toform a solid casting with the wire embedded within said casting.However, each of the above mentioned types of insulators are inherentlyincapable of producing the desired results or in accomplishing theultimate aim of lasting heating element.

More particularly, whereas a wire insulated with fused glass, has thedesirable quality of being heat conductive, but electricallynon-conductive, such an insulator by its nature will render theresistance wire almost totally inflexible. Consequently, even therelatively small degree of linear expansion and contraction that aresistance wire undergoes upon alternate heating and cooling will causean insulation of fused glass to break or crack, which gives rise to thepossibility of the wire short-circuiting through contact with the metalcasting in which the wire is embedded. Furthermore, where the insulatedwire is embedded in the casting in a zig-zag pattern, alternateexpansion and contraction movements of the wire will gradually wear awaythe inflexible insulation particularly at the nodes or bends of thezig-zag wire and thus, also will result short-circuiting of the wirethrough contact with the casting where the insulation has worn away.

Porcelain enamel and fire clay insulators are even more unsatisfactorythan fused glass, because, in addition to rendering the resistance wireinflexible with resultant likelihood of leading to short-circuiting ofthe wire as above explained, these latter insulating materials arerelatively poor heat conductors which fact, in turn, will greatly reduceheat efficiency of the unit.

Asbestos and mica are also unsatisfactory for insulating embeddedresistance wires within a casting. In the first place each of thesematerials is a heat insulator rather than a good heat conductor.Moreover, all of these ma- Furthermore, asbestos and mica display markedtendency to emit volatiles when subjected to the heat of molten metalduring pouring of the casting. The release of volatiles during pouringoperations will almost invariably result in the formation of cavitationsin the casting adjacent the Wire which can lead to hot spots in the wirewith consequent danger of the wire burning out at such points whererapid transfer of heat from the wire to the casting is impaired due topresence of cavitations or air pockets.

As above indicated, in the present heating unit I employ as an insulatoraround the resistance wire embedded within the casting, a tight,flexible, electrically insulative, braid of glass yarn. Glass yarn braidis, in the first place, a good heat conductor which insures rapidconduction of heat from the resistance wire to the body of the casting.Moreover, the braid, being flexible, allows for alternate expansion orcontracting movements of the resistance wire during normal use. Inshort, it has been found that expansion and contracting movements of theresistance wire will not break or crack the glass braid insulation, andbecause of the flexibility of the braid, no appreciable wearing awaythereof or dis due to relative movement between the braid and wire.

it is a further principal object and advantage of the present inventionto disclose a method of manufacturing eating units of the typehereinabove briefly disclosed. essence, the preferred practice of themethod comp the successive steps of wrapping or coiling a flexible glassbraided insulated resistance wire in zig-zag pattern arotmd a frame;positioning the frame and insulated Wire coiled therearound within acasting mold; and then pouring into the mold a molten mass of heatconductive material, such as molten aluminum alloy or the like, to forma solid casting.

Other objects and advantages of the present invention will becomeapparent upon reading the following specification and referring to theaccompanying drav gs in which similar characters of reference representcorresponding parts of each of the separate views.

in the drawings:

Fig. l is a side elevational View of the frame with the glass insulatedresistance wire coiled therearound in zi zag pattern.

Fig. 2 is a vertical sectional view in reduced scale of a suitable moldand showing the frame and insulated resistance wire coiled therearoundpositioned within the mold.

Fig. 3 is a side elevational view of a completed heating unit showingportions thereof broken away in sec- 1tion.

Fig. 4 is a vertical sectional view taken on line 4-4 of Fig. 3.

It is believed that the specific construction of the present heatingunit can be most readily understood by making reference to the preferredmethod of manufacturing said unit.

In manufacturing the unit, the resistance wire to be used, such asindicated at 10, is electrically insulated with a flexible braid ofglass yarn 11. Glass yarn braiding of electrically conductive wires is awell-known expedient in the art, and therefore the specific manner ofwrapping the glass yarn around the Wire to provide a tightly wound yetflexible braid does not in and of itself constitute a part of thepresent invention. It will sufice to remark that good results have beenobtained by wrapping the two braids each approximately .010 inch indiameter and with a standard under-two-over-two co truction. A braid ofthis character provides both excelle cal insulation as well asflexibility so that the wire can be coiled or bent or otherwise formedin desired pattern as will more fully appear hereinafter.

The braided insulated wire is positioned within a suitable mold asindicated at wherein a molten casting material is poured, so as tointimately embed the insulated wire within a solid casting of heatconductive material. In this connection, I provide a suitable wrappingframe, indicated generally at 12, about which is wrapped or coiled theinsulated wire prior to its insertion within the pouring mold. Morespecifically, the frame 12 may comprise an open metallic wire or rodframe which can be constructed in practically any shape or size to fitproperly within the mold. In the drawings, the frame 12 is shown asbeing rectangular in shape comprising parallel side frame pieces 13 andparallel end pieces 14 rigidly joined together. To insure that theresistance wire on a 16 carrier braider using frame and insulated wirewrapped therearound will be properly positioned Within the mold, Iprovide a plurality of longitudinal, transverse, and cross-wise spacerelements, which function to contact the interior walls of the mold andto maintain the frame body and resistance wire spaced inwardly from themold interior the desired distance and in proper position within saidmold. More specifically, longitudinal spacer elements 16 which proiectoutwardly beyond end pieces 14 of the frame body are adapted to contactthe top and bottom walls 17 and 18 of the mold when the frame structureis inserted within said mold. Transverse spacer elements 19, whichproject a short distance beyond side members 13 of the frame are adaptedto contact end walls 21 of the mold so as to maintain the framestructure in proper spaced relationship from the end walls when theframe is positioned within the mold cavity. Similarly, crosswise spacerelements 22 are adapted to contact the opposite side walls 23 of themold to maintain said frame in properly spaced relationship between saidside walls during pouring opcrations. I prefer to form all of the abovementioned spacer elements 1 6, 19 and 22 with sharply pointed ends so asto insure good casting results, and to prevent chilling effects aroundor adjacent to the ends of the spacer elements upon cooling of themolten casting material. I also prefer to make the frame structure 12out of the same material as is selected for the casting, so that in thefinished product the frame structure embedded within the solid casting2% will have the same coefficient of thermal expansion as the castingitself. This eliminates the danger of the frame structure expanding orcontracting to greater or lesser degree than the casting itself, whichcircumstance, if permitted to occur, would create unnecessary andpossibly deleterious effects with respect to the operation of the unit.Moreover, by making the frame structure of the same material as thecasting an intimate bond between the frame and the molten cast ispractically assured.

Before inserting the frame structure and the insulated resistance wirewrapped therearound within the mold it is, of course, desirable tothoroughly clean the exterior surface of the frame itself to eliminatethe possibility of dirt or other foreign matter, and particularlyvolatile mac ter, from causing cavitations forming in the casting. It isalso preferable to heat treat the braided resistance wire prior to itsinsertion within the mold cavity to drive off any residual volatilesfrom the braid.

After the frame with the insulated resistance wire has been properlypositioned within the mold, the molten casting material may be pouredinto the mold cavity through pouring sprue 26. The material selected tomake the casting should, of course, be one possessing the quality ofgood heat conductivity. Although the casting material selected shouldhave a sufficiently high melting point so as not to melt or volatalizedue to heat generated by the resistance wire, its melting point shouldbe substantially less than the fusing or melting point of the particularglass yarn insulation used. In this respect, it is important that themolten material when cast into the mold does not generate so much heatas will cause the glass yarn strands to fuse together with resultantloss of flexibility of the braided wire. 7

From the foregoing, it is evident that any number of different types ofmaterial may be employed to make the casting. I have found aluminumalloy to be one particularly satisfactory material in that it is notonly an excellent heat conductor, but, in its cast form, it isrelatively light in weight which is a desirable attribute in numerousapplications to which the heating unit may be put.

The actual casting of the molten metal may be accomplished in accordancewith conventional and accepted practices in the art employing suitablemolds which may be of more or less conventional construction and design.It willbe readily understood that particular care should be exercisedduring casting operations to avoid cavitations forming in the body ofthe casting so as to minimize the danger of establishing cavitated areasadjacent the wire where hot spots would occur. In making the casting,provision should also be made for extending lead wires 30 and 31(through which current can be supplied to the resistance wire)exteriorly of the mold. In this connection, the lead wires may be joinedto opposite ends of the resistance wire by means of conventional metaltube connectors, such as indicated at 33 and 34. When the wrap pingframe 12 is positioned in the mold cavity, the lead wires can beextended exteriorly of the cavity through suitable apertures 33a and 34aas shown in Fig. 2.

It is believed evident from the foregoing that a heating unitconstructed generally in accordance with the preferred method ofmanufacture hereinabove explained will comprise an open frame aboutwhich has been coiled or wrapped in zig-zag pattern a resistance wireelectrically insulated by a finely woven flexible braid of glass yarn,and which said frame and insulated resistance wire are intimatelyembedded within a solid casting of heat conductive material. As abovenoted, it is desirable that the frame and casting be made or" the samematerial so as to insure an intimate bond between the casting and theframe so that internal pressures within the casting will not occur dueto differences in coefiiciency of thermal expansion between the castingand the frame embedded therein.

It is also believed evident that a heating unit of the present type canbe cast in practically any desired shape and size. Although theembodiment of the invention shown in the drawings and describedhereinabove comprises a relatively fiat rectangular casting, it isreadily understandable that the cast units might be made with simple orcomponent curved surfaces, as well as in rounded, oval or irregularoutline configurations. In this regard, the wire wrapping frame, andalso the mold cavities in which the frames are adapted for positioning,can be shaped and proportioned so as to produce a finished casting ofpractically any desired shape, size, configuration or design.

Although the present invention has been described in some detail by wayof illustration and example, it is understood that certain changes andmodifications might be practiced within the spirit of the invention,which is limited only by the scope of the claims appended hereto.

1 claim:

1. An electrical heating unit comprising: a casting of heat conductivematerial; an electrically insulated resistance wire embedded within saidcasting and intimately surrounded by said casting material; theinsulation on said wire comprising a finely woven flexible braid ofelectrically insulated high heat conductive strands of material.

2. An electrical heating unit comprising: a casting of heat conductivemetal; an electrically insulated resistance wire embedded within saidcasting; the insulation around said wire comprising a flexible braid ofglass yarn.

3. An electrical heating unit comprising: a casting of heat conductivemetal; a zig-zag length of electrically insulated resistance wireembedded in said casting; the insulation on said wire comprising aflexible braid of glass yarn.

4. An electrical heating unit comprising: a winding frame of heatconductive material; an electrically insulated resistance wire coiledaround said frame; the insulation around said resistance wire comprisinga flexible braid of glass yarn; said frame and insulated wire coiledtherearound embedded within a casting of heat conductive material.

5. A heating unit according to claim 4 and whereiz frame and saidcasting are made of material having stantially the same coefficient ofthermal expansion.

6. Means for supporting and positioning a resistance wire within a moldcavity during pouring of molten casting material in said cavity,comprising; a wrapping frame defining a body portion about which aresistance wire may be wrapped, and means comprising spacer membersprojecting outwardly from the body portion of said for engaging theinterior surfaces of said mold cavity to maintain said body portion andresistance wire wrapped therearound spaced inwardly from the interiorsur 'aces of said mold cavity.

7. Means according to claim 6 and wherein said spacer members eachcomprise a pointed projection extending outwardly from said bodyportion.

8. In a method of producing an electrical heating unit of the typecomprising an electrical resistance wire embedded Within a solid castingof heat conductive material comprising the steps of: providing a moldcavity in which molten heat conductive material is to be poured to formthe casting; wrapping an electrically insulative glass yarn braidedresistance wire around a wrapping frame made of material havingsubstantially the same coeificiency of thermal expansion as the castingmaterial; positioning the frame and braided resistance wire wrappedtherearound within said mold cavity; and pouring the molten castingmaterial into said mold cavity to embed the frame and resistance wirewithin said casting material.

References Cited in the file of this patent UNITED STATES PATENTS890,858 Hadaway June 16, 1903 1,116,586 Hadaway Nov. 10, 1914 2,227,931Greenleaf Jan. 7, 1941 2,307,626 Kelly Jan. 5, 1943 2,403,022 ReimersJuly 2, 194-6 2,408,149 Miller et al Sept. 24-, 1946 FOREIGN PATENTS497,673 France Sept. 22, 1919

1. AN ELECTRICAL HEATING UNIT COMPRISING: A CASTING OF HEAT CONDUCTIVEMATERIAL; AN ELECTRICALLY INSULATING RESISTANCE WIRE EMBEDDED WITHINSAID CASTING AND INTIMATELY SURROUNDED BY SAID CASTING MATERIAL: THEINSULATION ON SAID WIRE COMPRISING A FINELY WOVEN FLEXIBLE BRAID OFELECTRICALLY INSULATED HIGH HEAT CONDUCTIVE STRANDS OF MATERIAL.